Much of the past work in the area of coherent optical processing has been performed since the development of the laser, currently a convenient source of highly intense coherent radiation. Coherent optical processing systems may take a variety of basic configurations, depending on the intended use. A prior art coherent processing system is schematically illustrated in FIG. 1.
In its operation a narrow collimated beam of coherent radiation from the laser source (L) passes through a beam expander (B) and is recollimated by the lens (L.sub.1). The resulting collimated beam of light illuminates an object transparency located in the input plane (P.sub.1). The object transparency contains in a two-dimensional format the information that is to be processed. The light amplitude distribution appearing in the back focal plane (P.sub.2) of lens L.sub.2 is proportional to the two-dimensional Fourier transform of the amplitude transmittance of the input object transparency. The actual processing of information is performed in plane P.sub.2. In this plane is located a spatial filter which is capable of modifying the amplitude and phase of the resulting Fourier spectrum associated with the input object transparency. Plane P.sub.2 is commonly referred to as the spatial filtering plane. The final lens (L.sub.3) in the system performs a Fourier transform of the modified object spectrum yielding in the output image plane (P.sub.3) the desired processed version of the original input object information.
One area in coherent optical processing where much research and effort has been devoted is in the realization of optical spatial filters. This is understandable in consideration of the fact that the spatial filter is perhaps the most important optical component in the system since it dictates the nature of the processing operation to be performed. In order that an optical spatial filter be useful for both light amplitude and phase manipulations, it is necessary that the filter have a complex-amplitude transmittance. Since conventional optical recording media respond only to light intensity, a real-positive quantity, the physical realization of complex spatial filters is a much involved process.
There are three well known techniques for the realization of complex spatial filters. In 1964 Vander Lugt developed a technique for recording a complex-valued spatial frequency function as a real-positive valued function on a spatial carrier frequency. With this technique, a complex filter function can thus be represented by an absorption pattern on conventional recording media like photographic film. The actual recording of the filter does require the use of an optical interferometer.
However, there are two principal limitations of the Vander Lugt technique. Firstly, the recording medium required for the synthesis of the filter must have high resolution capacity to accommodate the high spatial frequency content of the reference carrier. Secondly, the uncompensated output image in the output plane of the processor actually consists of four terms, with the desired filtered terms appearing off the axis of the optical processing system.
Another technique developed by Brown and Lohmann in 1966 avoids the complications involved in recording a complex filter using interferometric techniques and relaxes the requirements for the need of a high-resolution recording medium. This particular technique involves using a computer guided plotter to initially draw the desired complex filter in terms of a real-positive binary array representation. The resulting computer plot is then minified and recorded on photograhic film, yielding the desired complex filter. This technique, like the Vander Lugt technique, has the limitation that the filtered image appears off axis.
Another prior art technique, utilizes the polarization discrimination properties of Vectograph film to take advantage of the two-channel capacity inherent in polarized light for separate carriers of information. This technique was initially employed by Holladay and Gallatin in 1966 for controlling the sign of a real filter function.
Based on a theoretical analysis of the interaction of polarized light with Vectograph film, Marathay in 1969 derived general solutions for the realization of both real bipolar and complex spatial filters. This latter technique has the advantage that only one image term appears in the output plane of the processor, namely the desired filtered output, and that it appears on the axis of the optical processing system. The principal disadvantage of Vectograph film is that four sheets of film are required for the synthesis of a complex filter. Also, Vectograph film has a spatial cutoff frequency of about 250 lpm and can be recorded on only once.
There are a number of application areas where coherent optical processing offers significant advantages over their electronic counterparts. Image enhancement, pattern recognition, word recognition, fingerprint identification, Chromosome spread detection, communication processing, earth resources analysis, land-use analysis, broadband radar signal processing, sonar signal processing, seismology, antenna pattern analysis, and signal analysis are typical of such applications.
However, many new application areas would open up to coherent optical processing if a reusable write-read-erase optical medium were available. The rapidly advancing field of optical materials research offers the possible solution for the need of active recording media in coherent optical processing systems. During the past few years, new optical materials have demonstrated their potential as reusable recording media, but not without exhibiting their limitations also. This is evidenced by the proliferation of articles appearing in print addressing the need for active recording media.
One particular medium, namely the alkali-halide crystal containing anisotropic color centers, offers the potential for satisfying the need of a reusable write-read-erase recording medium in coherent optical processing systems. In particular, these materials which have come to be known as photodichroic crystals can be used in spatial filtering. Advantageously only two photodichroic crystal plates are required for the synthesis of complex spatial filters, not four as is the case with Vectograph film. In addition, photodichroic crystals have spatial frequency cutoffs greater than 1000 lpm.